CI: Install Android NDK r23c explicitly

It has just been removed from the Ubuntu 20.04 default install,
breaking our CI setup.

Also, sets Emscripten version to 3.1.39, as done upstream.
Newer versions actually break dynamic library support.

.github/workflows/ci.yml

@@ -65,7 +65,7 @@ jobs:
             platform: android
             artifact-name: godot-cpp-android-arm64-release
             artifact-path: bin/libgodot-cpp.android.template_release.arm64.a
-            flags: ANDROID_NDK_ROOT=$ANDROID_NDK_LATEST_HOME arch=arm64
+            flags: arch=arm64
             run-tests: false
             cache-name: android-arm64
 
@@ -88,7 +88,7 @@ jobs:
 
     env:
       SCONS_CACHE: ${{ github.workspace }}/.scons-cache/
-      EM_VERSION: 3.1.45
+      EM_VERSION: 3.1.39
       EM_CACHE_FOLDER: "emsdk-cache"
 
     steps:
@@ -108,11 +108,12 @@ jobs:
         with:
           python-version: '3.x'
 
-      - name: Linux dependencies
-        if: ${{ matrix.platform == 'linux' }}
-        run: |
-          sudo apt-get update -qq
-          sudo apt-get install -qqq build-essential pkg-config
+      - name: Android dependencies
+        if: ${{ matrix.platform == 'android' }}
+        uses: nttld/setup-ndk@v1
+        with:
+          ndk-version: r23c
+          link-to-sdk: true
 
       - name: Web dependencies
         if: ${{ matrix.platform == 'web' }}
@@ -121,16 +122,16 @@ jobs:
           version: ${{env.EM_VERSION}}
           actions-cache-folder: ${{env.EM_CACHE_FOLDER}}
 
-      - name: Install scons
-        run: |
-          python -m pip install scons==4.0.0
-
       - name: Setup MinGW for Windows/MinGW build
         if: ${{ matrix.platform == 'windows' && matrix.flags == 'use_mingw=yes' }}
         uses: egor-tensin/setup-mingw@v2
         with:
           version: 12.2.0
 
+      - name: Install scons
+        run: |
+          python -m pip install scons==4.0.0
+
       - name: Generate godot-cpp sources only
         run: |
           scons platform=${{ matrix.platform }} build_library=no ${{ matrix.flags }}